When effecting deep penetration welds using a laser beam, a plasma plume comprising vaporised metal and ionized welding gas is usually established over the welding zone between the workpieces being welded and the head of the welding apparatus. This plasma tends to absorb part of the energy of the laser beam which has to pass through the plasma before it impinges on the welding zone. In this way the amount of energy from the laser beam available for welding is reduced. It has been proposed to provide a tube terminating between the head of the welding apparatus and the workpieces through which a transverse jet of gas is passed to act on and to destroy the plasma. However, a disadvantage of such a system is that ambient air is entrained by the transverse jet and brought into the welding zone tending to cause porosity in the weld and/or changes in composition due to oxidation.
The invention seeks to overcome this disadvantage and accordingly resides in laser welding apparatus comprising: a body including a first passage defining an optical path for a laser beam; means for directing a laser beam along said passage and for focussing such beam to a position outside the body closely adjacent to a terminal portion of said passage at which workpieces to be welded together are, in use, located; the body including a second passage having an outlet in said terminal portion of the first passage and which is adapted to direct a jet of shielding gas transversely of said first passage to impinge on plasma formed within said terminal portion during welding; and the wall of said terminal portion of the first passage having an aperture so disposed as to permit said jet to sweep plasma away from the welding zone through the aperture.
The term `shielding gas` is used herein to mean a gas which in use tends to exclude ambient air from the welding zone and which does not react with any constituents in the weld pool or otherwise adversely affect the weld. A preferred gas is helium.
It is a feature of the invention that the terminal portion of the first passage in use extends to a position closely adjacent to the workpieces thereby assisting in the shielding of the welding zone from air entrainment. Furthermore, the disposition of the outlet from said second passage is such that the transverse jet which it directs can be made to impinge on the plasma substantially without bringing air into the welding zone. The apparatus may also comprise shielding means -- i.e. means adapted to restrict the access of ambient air to the welding zone -- fast with the body and extending downstream of the apperture in the wall of the terminal portion of the first passage in the sense of the intended direction of flow of shielding gas and plasma through that aperture.
Either transmitting optics or reflecting optics can be used for focussing the laser beam and the focussing means may comprise a lens supported in the first passage of the body or a concave mirror supported externally of the body. In the former case the body may include an inlet for pressurised shielding gas which opens into the first passage between the lens and the terminal portion of the passage, so that a stream of such gas can be directed through the passage to further shield the welding zone. Such gas may also assist in cooling the lens and protecting the lens from fume and spatter from the welding zone.
The apparatus may further comprise clamping means for clamping workpieces to be welded together to a support structure, the clamping means comprising surfaces complementary to surfaces of the aforesaid body whereby said surfaces of the body and clamping means can cooperate to restrict the access of ambient air to the welding zone. The body and clamping means may be adapted for relative movement, whereby, in use, a continuous weld seam can be produced between the workpieces clamped by the clamping means, in which case the clamping means may comprise elongate surfaces complementary to lateral surfaces of the body whereby said surfaces of the body and clamping means can cooperate to restrict the access of ambient air to the welding zone throughout such relative movement. In any such embodiment the outlet from the second passage in the body is preferably so disposed with regard to the direction of the relative movement for which the body and clamping means are adapted that the transverse jet of shielding gas can be directed along the most newly welded areas of such a seam.